CBSE Class 11 Biology Chapter 4 Notes: Animal Kingdom

Chapter Introduction

Animal Kingdom is Chapter 4 of CBSE Class 11 Biology and belongs to the Diversity in the Living World unit. This chapter classifies the animal kingdom based on fundamental features of body design — symmetry, germ layers, coelom, segmentation, and notochord. You will study all major phyla from the simplest (Porifera) to the most complex (Chordata), including the five vertebrate classes.

This chapter is large and detail-heavy. The key to mastering it is to understand the basis of classification and then fit each phylum into that framework, rather than memorising each phylum in isolation.

Key Definitions

Detailed Concepts

1. Basis of Classification

Animals are classified based on the following fundamental features:

2. Phyla of the Animal Kingdom

Phylum Porifera (Sponges)

• Symmetry: Mostly asymmetrical; some radially symmetrical.
• Germ layers: Cellular level of organisation (no true tissues).
• Coelom: Absent.
• Body plan: Pore-bearing; water enters through ostia and exits through the osculum.
• Body wall: Has two layers — pinacoderm (outer) and choanoderm (inner); mesoglea (non-cellular) in between.
• Special cells: Choanocytes (collar cells) line the spongocoel — unique to sponges; amoebocytes in mesoglea transport nutrients; pinacocytes form the outer layer.
• Skeleton: Made of spicules (calcium carbonate or silica) or spongin fibres.
• Reproduction: Asexual (fragmentation, gemmules); sexual (internal fertilisation); hermaphrodite.
• Habitat: Marine (mostly) and freshwater.
• Examples: Sycon, Spongilla (freshwater), Euspongia (bath sponge), Euplectella (Venus flower basket).

Phylum Coelenterata (Cnidaria)

• Symmetry: Radial.
• Germ layers: Diploblastic (ectoderm, endoderm, mesoglea in between).
• Coelom: Absent (acoelomate).
• Body plan: Two basic forms — Polyp (sessile, cylindrical, e.g., Hydra) and Medusa (free-swimming, umbrella-shaped, e.g., jellyfish). Some show alternation of these forms (metagenesis).
• Special cells: Cnidoblasts (stinging cells) contain nematocysts for defence and prey capture — unique to this phylum.
• Digestion: Extracellular and intracellular (incomplete digestive tract — single opening serves as both mouth and anus).
• Reproduction: Asexual (budding in polyps); sexual (in medusae).
• Examples: Hydra (freshwater), Physalia (Portuguese man-of-war), Aurelia (jellyfish), Adamsia (sea anemone), Corals (form coral reefs).

Phylum Ctenophora (Comb Jellies)

• Symmetry: Biradial.
• Germ layers: Diploblastic.
• Body plan: Eight external rows of ciliated comb plates (ctenes) for locomotion.
• Special features: Bioluminescence; no nematocysts (have colloblasts/adhesive cells instead).
• Digestion: Extracellular and intracellular.
• Examples: Pleurobrachia, Ctenoplana.

Phylum Platyhelminthes (Flatworms)

• Symmetry: Bilateral.
• Germ layers: Triploblastic.
• Coelom: Acoelomate (no body cavity).
• Body plan: Dorsoventrally flattened body; organ-system level of organisation.
• Digestion: Incomplete (single opening); some parasitic forms lack digestive system (absorb nutrients through body surface).
• Excretion: Flame cells (solenocytes) — unique excretory structure.
• Reproduction: Mostly hermaphrodite; high regeneration capacity; some reproduce by fission.
• Habitat: Free-living (Planaria in freshwater) or parasitic (Taenia/tapeworm, Fasciola/liver fluke).
• Examples: Dugesia (Planaria), Taenia solium (tapeworm), Fasciola hepatica (liver fluke).

Phylum Aschelminthes (Nematoda — Roundworms)

• Symmetry: Bilateral.
• Germ layers: Triploblastic.
• Coelom: Pseudocoelomate.
• Body plan: Circular in cross-section; organ-system level of organisation.
• Digestion: Complete digestive tract (mouth and anus both present).
• Body wall: Non-segmented; covered by a tough, resistant cuticle.
• Sexes: Separate (dioecious — distinct males and females); females often larger.
• Examples: Ascaris (roundworm), Wuchereria (filarial worm — causes elephantiasis), Ancylostoma (hookworm), Enterobius (pinworm).

Phylum Annelida (Segmented Worms)

• Symmetry: Bilateral.
• Germ layers: Triploblastic.
• Coelom: Coelomate (true coelom).
• Body plan: Metameric segmentation (body divided into similar segments/metameres); organ-system level.
• Digestion: Complete digestive tract.
• Locomotion: By setae (bristles) or parapodia (fleshy lateral appendages).
• Circulatory system: Closed type (blood confined to vessels).
• Excretion: Nephridia.
• Nervous system: Ventral nerve cord; dorsal cerebral ganglia.
• Examples: Nereis (aquatic, parapodia), Pheretima (earthworm, setae), Hirudinaria (leech, suckers).

Phylum Arthropoda (Jointed Appendages)

• Symmetry: Bilateral.
• Germ layers: Triploblastic.
• Coelom: Coelomate (true coelom, reduced; body cavity is haemocoel filled with blood).
• Body plan: Jointed appendages; exoskeleton of chitin; body divided into head, thorax, abdomen (some have cephalothorax); organ-system level.
• Circulatory system: Open type (blood flows through body cavity/haemocoel).
• Excretion: Malpighian tubules (insects), green glands (crustaceans), coxal glands (arachnids).
• Respiration: Gills (crustaceans), trachea (insects), book lungs (scorpions, spiders), book gills (Limulus).
• Moulting: Ecdysis — periodic shedding of exoskeleton for growth.
• Largest phylum — about 80% of all animal species.
• Classes: Crustacea (crabs, prawns), Arachnida (spiders, scorpions), Myriapoda (centipedes, millipedes), Insecta (largest class — butterflies, beetles, bees, cockroaches).
• Examples: Palaemon (prawn), Cancer (crab), Limulus (king crab), Apis (honeybee), Bombyx (silkworm), Laccifer (lac insect).

Phylum Mollusca (Soft-bodied Animals)

• Symmetry: Bilateral (some secondarily asymmetrical, e.g., snails).
• Germ layers: Triploblastic.
• Coelom: Coelomate (reduced coelom).
• Body plan: Soft body; often enclosed in a calcareous shell; body divided into head, visceral mass, foot, and mantle (a fold of body wall that secretes the shell).
• Digestion: Complete; radula (rasping tongue-like organ) in most — unique to molluscs.
• Circulatory system: Open type (except cephalopods — closed).
• Respiration: Gills (ctenidia) in aquatic forms; pulmonary sac in terrestrial forms.
• Excretion: Kidney-like metanephridia.
• Examples: Pila (apple snail), Unio (freshwater mussel), Octopus, Sepia (cuttlefish), Loligo (squid), Aplysia (sea hare), Dentalium (tusk shell), Chaetopleura (chiton).

Phylum Echinodermata (Spiny-skinned Animals)

• Symmetry: Radial (adults); bilateral (larvae).
• Germ layers: Triploblastic.
• Coelom: Coelomate.
• Body plan: Unsegmented; endoskeleton of calcareous ossicles (spines); water vascular system (tube feet) for locomotion, respiration, and food capture.
• Digestion: Complete; anus absent in some.
• Special features: Deuterostomes (closer to chordates developmentally); regeneration common.
• Examples: Asterias (starfish), Ophiura (brittle star), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), Holothuria (sea cucumber).

Phylum Hemichordata

• Symmetry: Bilateral.
• Germ layers: Triploblastic.
• Coelom: Coelomate.
• Body plan: Worm-like; body divided into proboscis, collar, and trunk.
• Special features: Gill slits present; stomochord (not a true notochord); considered a connecting link between non-chordates and chordates.
• Examples: Balanoglossus (acorn worm), Saccoglossus.

Phylum Chordata

Chordates possess four fundamental features at some stage of their life:

1. Notochord: A longitudinal, flexible rod of vacuolated cells; provides skeletal support; replaced by vertebral column in vertebrates. 2. Dorsal hollow nerve cord: A tubular nerve cord dorsal to the notochord; forms the central nervous system (brain and spinal cord). 3. Pharyngeal gill slits: Paired openings in the pharynx; used for filter-feeding or respiration in aquatic forms; modified in terrestrial forms. 4. Post-anal tail: Extension of the body beyond the anus; supported by the notochord or vertebrae.

Subphyla:

Chordate Features vs Vertebrate Features:

• All vertebrates are chordates, but not all chordates are vertebrates (urochordates and cephalochordates lack a vertebral column).

3. Vertebrata — Classes Comparison

Key distinctions within Pisces:

4. Phyla Comparison — Key Features

Important Diagrams and Labelling

1. Cross-section of Coelom Types: Draw three cross-sections showing (a) Acoelomate (solid mesoderm, e.g., flatworm), (b) Pseudocoelomate (mesoderm as scattered pouches, e.g., roundworm), (c) Coelomate (mesoderm lines both sides of cavity, e.g., annelid). Label ectoderm, mesoderm, endoderm, gut, and coelom in each.

2. Hydra (Coelenterata): Draw Hydra showing body column, tentacles, basal disc, mouth, and bud. Label the cnidoblasts on tentacles.

3. Four Chordate Features: Draw a simple chordate body diagram showing (1) notochord (dorsal rod), (2) dorsal hollow nerve cord above it, (3) pharyngeal gill slits on the sides, (4) post-anal tail. Label all four.

4. Earthworm (Annelida): Draw an earthworm showing prostomium, segments, setae, clitellum, and anus. Label each.

5. Fish Body Plan: Draw a bony fish showing terminal mouth, operculum, paired fins (pectoral and pelvic), median fins (dorsal, anal, caudal), lateral line, and scales.

Common Mistakes to Avoid

Exam Tips

1. For 1-mark questions: Know the unique features of each phylum — choanocytes (Porifera), nematocysts (Coelenterata), flame cells (Platyhelminthes), jointed appendages (Arthropoda), water vascular system (Echinodermata), notochord (Chordata). Know the four chordate features. 2. For 2-mark questions: Differentiate between any two phyla, or between acoelomate/pseudocoelomate/coelomate, or between protostomes and deuterostomes. 3. For 3-mark questions: Write about a phylum with its key features, symmetry, coelom type, digestion type, and examples. Or explain the basis of classification with all features. 4. For 5-mark questions: Prepare the complete phyla comparison table. Or write about the classes of Vertebrata with their features and examples. Or discuss the four chordate features with their significance. 5. Draw diagrams: The coelom cross-sections and the chordate body plan are the most commonly asked diagrams. Always label all layers. 6. Use NCERT language: Write "cellular level of organisation" (not "no organisation"), "incomplete digestive tract," "open/closed circulatory system," "retrogressive metamorphosis," "metameric segmentation."

Quick Revision Notes

• Basis of classification: Symmetry (asymmetrical, radial, bilateral), germ layers (diploblastic, triploblastic), coelom (acoelomate, pseudocoelomate, coelomate), segmentation, notochord.
• Porifera: Asymmetrical, cellular level, ostia → spongocoel → osculum, choanocytes, spicules/spongin. Sycon, Spongilla.
• Coelenterata: Radial, diploblastic, acoelomate, cnidoblasts, polyp/medusa. Hydra, Aurelia, Adamsia.
• Ctenophora: Biradial, diploblastic, comb plates, bioluminescence. Pleurobrachia.
• Platyhelminthes: Bilateral, triploblastic, acoelomate, flame cells, flat body. Planaria, Taenia, Fasciola.
• Aschelminthes: Bilateral, triploblastic, pseudocoelomate, dioecious, cuticle. Ascaris, Wuchereria.
• Annelida: Bilateral, triploblastic, coelomate, metameric segmentation, closed circulation, nephridia. Nereis, Pheretima, Hirudinaria.
• Arthropoda: Largest phylum. Chitinous exoskeleton, jointed appendages, open circulation, moulting. Prawn, crab, spider, insects.
• Mollusca: Soft body, mantle, shell, radula. Pila, Octopus, Sepia.
• Echinodermata: Radial (adults), bilateral (larvae), water vascular system, tube feet, deuterostomes. Asterias, Echinus.
• Hemichordata: Proboscis-collar-trunk, gill slits, stomochord. Balanoglossus.
• Chordata: Notochord + dorsal hollow nerve cord + pharyngeal gill slits + post-anal tail. Urochordata (larval notochord), Cephalochordata (lifelong notochord), Vertebrata (vertebral column).
• Vertebrata: Pisces (2-chambered, gills, scales), Amphibia (3-chambered, moist skin, non-amniotic), Reptilia (3-chambered, dry scales, amniotic), Aves (4-chambered, feathers, warm-blooded), Mammalia (4-chambered, hair, mammary glands, warm-blooded).